Influence of superconductivity on magnetic properties of superconductor/ferromagnet epitaxial bilayers

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Abstract

Single crystal Fe/Nb (110) bilayers with a Nb thickness dNb in the range from 250 to 650 Å and with a Fe thickness dFe in the range from 7 to 27 Å were prepared using MBE techniques. Magnetization measurements showed the existence of a magnetically `dead' Fe-interface-layer with a thickness dNM varying strongly with minor modifications of the growth conditions. For bilayers with a small magnetic layer thickness dM and a small dNM the FMR measurements revealed an anomalous decrease of the effective magnetization 4πMeff of the ferromagnetic Fe-layer below the superconducting transition temperature Tc of Nb. Additional magnetization measurements using a SQUID magnetometer indicated that the anomalous decrease is indeed due to a decrease of the magnetization M of the Fe layer. The absolute change of 4πMeff below Tc depends strongly on the actual values of dM and dNM: upon decreasing both dM and dNM the effect increases considerably. We discuss two possible mechanisms for the anomalous temperature dependence of 4πMeff below Tc: The first mechanism is a new type of cryptoferromagnetism, i.e. a spatial modulation of ferromagnetic order due to a modification of the RKKY-interaction in the superconducting state. The second mechanism, which at the moment cannot be excluded, is a spatial modulation of the spin direction in the Fe-layer caused by the demagnetizing field in the geometrically rough interface.

Introduction

The study of the mutual interaction of superconductivity and ferromagnetism in dilute magnetic alloys and intermetallic compounds attracted considerable attention during the last 30 years (see, e.g., [1]). Initial interest in these phenomena arose after the experiments of Matthias et al. 2, 3, who measured superconducting transition temperatures and magnetic ordering temperatures of intermetallic compounds containing magnetic impurities. These experiments clearly revealed the destructive influence of magnetic impurities on superconductivity. At the same time, the results of these measurements indicated that superconductivity and ferromagnetism might coexist. At the first glance this seems to be in contradiction with the BCS theory, because in the simplest case the conduction electron spin-susceptibility χS(0)=0 in the superconductor and the strong internal exchange field in the ferromagnetic state, which tends to align the conduction electron spins, are incompatible. Different explanations were suggested, proposing that superconductivity may coexist with a ferromagnetic state in magnetic domains smaller than the superconducting coherence length (see, e.g., [4]). This so-called cryptoferromagnetic state arises due to antiferromagnetic long-range RKKY-type interactions between magnetic ions in the superconducting state from electronic correlations in the Cooper-pair condensate. Thus, principally the coexistence of a superconducting and a ferromagnetic state in dilute magnetic alloys is possible, if the ferromagnetic state is reconstructed in an appropriate way.

The mutual influence of superconductivity and ferromagnetism acquires new peculiarities in artificially prepared multilayers combined of superconductors and ferromagnets (S/F). Recently, we have studied the effect of ferromagnetic Fe-layers on the superconducting parameters of a Nb layer in sputtered Fe/Nb/Fe trilayers 5, 6. In the present paper we show for the first time that, vice versa, the ferromagnetic state of Fe can also be modified by the superconducting state of Nb in a subtle way. This effect was observed on high quality (110) single crystal Nb/Fe bilayers prepared by MBE techniques. Preliminary results of the present study have been published in Ref. [7].

The remainder of this paper is organized as follows. In Section 2we provide a brief outline of the sample preparation and X-ray characterization and describe our data on the superconducting transition temperatures, and on resistivity and magnetization of the Fe/Nb bilayers. The ferromagnetic resonance (FMR) measurements are presented in Section 3, followed by a discussion of all results obtained in this study in Section 4. The main results are summarized in Section 5.

Section snippets

Sample preparation

The Nb/Fe bilayers on Al2O3 (112̄0) substrates were prepared using a molecular beam epitaxy (MBE) system (for details about this equipment see, e.g., [8]). The vacuum base pressure of the machine is 5×10−9 Pa, the working pressure is 2×10−8 Pa. Nb was evaporated by electron beam from a 14 cm3 crucible. Evaporation rates of 0.5 Å/s were found to be optimal for the growth of high quality single crystal Nb(110) films with a superconducting transition temperature of Tc=8.5 K. Fe was deposited by an

FMR results and analysis

Ferromagnetic resonance (FMR) experiments were carried out at 9.4 GHz in the temperature range from 1.7 K to 380 K using the ESR spectrometer B-ER 418S (Bruker AG). FMR signal was observed for the samples with dM≥9 Å. The angular dependence of the spectra was studied with both the DC magnetic field and the high frequency field lying in the film plane. The (110) surface of our thin films contains the magnetically `easy' {001}, `hard' {111} and `intermediate' {110} axes of bulk Fe.

For T≥320 K a

Discussion

The most interesting result of the present study is the anomalous low temperature behavior of the resonance fields for H‖[110] (see Fig. 7). We clearly notice that with decreasing the temperature below the superconducting transition at Tc the separation between the H0u and H0a values increases for the samples MBE 742, MBE 808, MBE 856(8). Only for the sample MBE 640 the resonance fields are temperature independent within the error bars. The observed temperature dependence of H0u and H0a cannot

Summary

The main purpose of the present paper was the study of the influence of the superconducting state of Nb on the ferromagnetic state of Fe in single crystalline Fe/Nb bilayers. The FMR data clearly indicate a decrease of the effective magnetization 4πMeff of the Fe-layer below the superconducting transition temperature Tc of the Nb-layer. This is supported by direct magnetization measurements using a SQUID magnetometer. We tentatively interpreted the results as the first experimental evidence for

Acknowledgements

The authors would like to thank Prof. G.G. Khaliullin and Prof. M.M. Zaripov for helpful discussion and Prof. L.R. Tagirov for his participation in this study during the early stage. This work was supported by the Deutsche Forschungsgemeinschaft (DFG-ZA161/6-2) and by the Russian Fund for Fundamental Research (Project 96-02-16332a), which are gratefully acknowledged. One of us (I.A.G.) acknowledges NATO support under the Collaborative Research Grant HTECH.CRG961371.

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